mx=0
s = input()
a = [s[0],s[1],s[2],s[3],s[0:2],s[1:3],s[2:4],s[0:3],s[0:3],s[1:4], s]
for j in range(len(a)):
    n= int(a[j])
    c = 0
    for i in range(1,n+1):
        if n%i ==0:
            c = c+1
    if c==2:
        mx=max(mx,n)
print(mx)

